Various air filters have been designed in the past for cleaning, purifying or both, a volume of air. For example, U.S. Pat. No. 2,145,049 to Heuberger relates to an air purifier. The species of FIG. 4 is considered pertinent and is generally made up of a series of concentric filter elements of similar or dissimilar material in tandem relationship. The filter elements are supported by a housing. A motor driven fan is connected to one end of the housing for generating an air flow to be purified through the filters. U.S. Pat. No. 3,252,270 to Pall et al. discloses a double-stage oil filter for attachment to a pipeline arrangement. The single housing configuration of the filter in FIG. 1 is considered pertinent. U.S. Pat. No. 4,322,230 to Schoen et al. concerns a double-stage disposable air filter of unitized construction. The filter housing is provided with capped inlet and outlet ports until the filter is connected to a pipeline arrangement for filtering an air flow. U.S. Pat. No. 4,477,270 to Tauch discloses a portable three-stage air filter of cylindrical-shaped configuration for attachment to a cabinet housing in some fashion. The filter is generally comprised of a series of three concentric filter elements, the inner element being composed of densified carbon particles and two outer polyurethane foam shells. U.S. Pat. No. 4,339,250 to Thut relates to a self-contained, floor-disposed three-stage air filter of cylindrical shape for progressively cleaning and purifying an enclosed air space. The filter is generally made up of an outer filter element of cloth material, an intermediate filter element of fiberglass, and an inner filter element of activated granular carbon of a predetermined density. However, none of the aforediscussed references, whether taken alone or in any combination, remotely suggest an improved double-stage air filter for attachment between a spaced double wall structure in single or multiple arrayed fashion in meeting the filtered air requirements of a controlled area. The filter is provided with a casing extending between aligned openings in the walls and attached to the inlet wall of the wall structure. A combined plate and perforated sleeve arrangement is inserted in the casing and connected to the outlet wall of the structure. A concentric inner and outer filter arrangement is disposed about the sleeve within the casing so as to provide a unique and efficient air flow filtration path from the sleeve interior adjacent the outer wall inlet opening through the filter arrangement and the novel slot means of the plate means to the inner wall outlet opening so as to provide a cleaned and purified air flow to the controlled area.